The present invention relates to an A/D conversion apparatus for converting an analog input signal into a digital signal and, more particularly, to an A/D conversion apparatus to be incorporated in a microcomputer.
In recent years, with development of LSI technology, a microcomputer has been highly integrated, so that several kinds of peripheral hardware are mounted on a single chip. Particularly, in an A/D converter, which is indispensable in the fields of such as automobile control and AC servo control, the demand for the microcomputers incorporating the A/D converter has been greatly enhanced.
In a previously known microcomputer incorporating an A/D converter, which has a plurality of analog signal input terminals, generally, a scanning operation of successively A/D converting the plural analog input terminals is executed. For example, in the case where the A/D converter comprises four analog input terminals of AN.sub.0 .about.AN.sub.3, and four registers REG.sub.0 .about.REG.sub.3 for storing the result of the A/D conversion, an A/D conversion is successively conducted on the four terminals of AN.sub.0 .about.AN.sub.3 in the order of AN.sub.0 .fwdarw.AN.sub.1 .fwdarw.AN2.fwdarw.AN.sub.3, and the conversion results thus obtained are stored in the registers REG.sub.0 .about.REG.sub.3 corresponding to the input terminals AN.sub.0 .about.AN.sub.3, respectively. Upon completion of the A/D conversion for the AN.sub.0 .about.AN.sub.3, the A/D converter issues an interruption request signal to a central processing unit (hereinafter referred to as "CPU") within the microcomputer. When the CPU completes reading the conversion results from the REG.sub.0 .about.REG.sub.3, the A/D conversion is started again. Thereafter, the operation described above will be repeated.
However, the A/D converter described above has the following defects. The analog signals at the input terminals are successively A/D-converted through the scanning operation therefor in a fixed order, but their priority or their conversion order cannot be changed. In order to give a priority to a given input terminal or change the conversion order, an analog multiplexer or the like should be provided externally of the input terminals so that it changes the connections of the input terminals. This gives rise to an increase of the production costs.
Further, the A/D conversion of a certain analog input which is to be realized at a high speed may be kept waiting owing to the conversion of other analog input(s). This gives rise to inconveniences such as delay of control timings which leads to reduction of the performance of an application system.